| Summary: | Transparent oxyfluoride glass–ceramics containing NaGdF<sub>4</sub> nanocrystals were prepared by melt-quenching and doped with Er<sup>3+</sup> (0.5 mol%) and different amounts of Yb<sup>3+</sup> (0–2 mol%). The selected dopant concentration the crystallization thermal treatments were chosen to obtain the most efficient visible up-conversion emissions, together with near infrared emissions. The crystal size increased with dopant content and treatment time. NaGdF<sub>4</sub> NCs with a size ranging 9–30 nm were obtained after heat treatments at T<sub>g</sub> + 20–80 °C as confirmed by X-ray diffraction and high-resolution transmission electron microscopy. Energy dispersive X-ray analysis shows the incorporation of rare earth ions into the NaGdF<sub>4</sub> nanocrystals. Near-infrared emission spectra, together with the up-conversion emissions were measured. The optical characterization of the glass–ceramics clearly shows that Er<sup>3+</sup> and Yb<sup>3+</sup> ions are incorporated in the crystalline phase. Moreover, visible up-conversion emissions could be tuned by controlling the nanocrystals size through appropriated heat treatment, making possible a correlation between structural and optical properties.
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